[Bug target/100799] Stackoverflow in optimized code on PPC

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100799

--- Comment #28 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Peter Bergner from comment #27)
> So I looked closer at what the failure mode was in this PR (versus the one
> you're seeing with flexiblas).  As in your case, there is a mismatch in the
> number of parameters the C caller thinks there are (8 args, so no param save
> area needed) versus what the Fortran callee thinks there are (9 params which
> include the one hidden arg, so there is a param save area).  The Fortran
> function doesn't actually access the hidden argument in our test case above,
> in fact the character argument is never used either.  What I see in the rtl
> dumps is that *all* incoming args have a REG_EQUIV generated that points to
> the param save area (this doesn't happen when there are 8 or fewer formal
> params), even for the first 8 args that are passed in registers:

Yes, so it is the backend that told function.cc that there is a parameter save
area and it should be adding REG_EQUIV notes.  So, the idea would be that for
the case we talk about (<= 8 normal arguments, then only unused
DECL_HIDDEN_STRING_LENGTH ones) that the backend would also say that there is
no parameter save area, basically pretend there are <= 8 arguments.

> > Doing the workaround on the caller side is impossible, this is for calls
> > from C/C++ to Fortran code, directly or indirectly called and there is
> > nothing the compiler could use to guess that it actually calls Fortran code
> > with hidden Fortran character arguments.
> As a HUGE hammer, every caller could always allocate a param save area. 
> That would "fix" the problem from this bug, but would that also fix the bug
> you're seeing in flexiblas?

Most likely yes.  Though of course that is way too high price to pay, even with
some non-default option.  If we can't workaround it in the backend just on the
callee side of calls which have the unused hidden string length arguments, then
better no changes
on the GCC side.